Petrology

3696

Petty

lines in the United States has become so com-
plete that it is possible to pump oil produced
in the central part of Texas to the refineries
at the Atlantic seaboard, a distance of over
2,500 miles.

Since oil is a prime requisite of belligerents,
during World War II attention was focused
on location and production of the world's oil
fields. In the U. S., in 1942, the petroleum
industry was organized for war effort in co-
operation with the petroleum co-ordinator,
Harold L. Ickes.

Petrology, Petrography, or Lithology,
the science of rocks, a branch of geology
which has many relations with the cognate
science of mineralogy. It is concerned princi-
pally with the composition, structure, inter-
pretation, and classification of rock. Much
can be learned about rocks by simple naked-
eye inspection, or with the aid of a pocket
lens. For the examination of the finer grained
rocks, and of the mineral properties which
characterize the minute crystals and frag-
ments of which most rocks are composed, re-
course must be had to more refined methods
of investigation. The rock may be chemically
analyzed, and a knowledge of its bulk com-
position never fails to indicate in which cate-
gory it is to be placed, provided that its
principal mineral components and its macro-
scopic characters are already known. But an
even more potent auxiliary is the microscope,
The great rock groups, as employed in most
works on petrology, are sedimentary rocks,
igneous rocks, metamorphic rocks.

Sedimentary rocks consist of broken,
rounded fragments (e,g., the conglomerates),
or of small, worn sand grains (e.g., sand-
stones, grits, arkoses), or of the finest muddy
and clayey silts (clay, shale, marls). As a
group they have certain well-defined char*
acters. They are mostly divided up into thin
sheets or beds, which have parallel upper and
under surfaces; they consist of broken dibris
of pre-existing rocks, which, having accumu-
lated in seas, lakes and upon land, have been
subsequently subjected to pressure and pressed
mto solid form.

Igneous rocks form another well defined
group, produced as a result of volcanic and
eruptive forces. Omitting the sedimentary
and clastic ash beds, they are crystalline, and
have at one time been in a state of fusion,
from which they have cooled more or less
slowly. Their structure and the minerals of
which they are composed depend mainly on
two factors-^;., the chemical composition
of the magma, or molten mass from which

they proceeded, and the physical conditions
under which they solidified.

Metamorphic rocks, of which the best
known are the schists and gneisses, very gen-
erally have a banded or foliated appearance
and a crystalline structure. See ROCKS. Spec-
ial works on the subject are Rutley's Study
of Rocks; Harker's Petrology for Students
(IQOS) ; Iddings* Igneous Rocks (1909).

Petronel, an ancient and clumsy form of
pistol.

Pefcronius, Gaius (d. c. 66 A.D.), surnamed
Arbiter, from his supposed identity with the
Petronius whom Tacitus calls 'arbiter ele-
gantish at the court of Nero, is generally be-
lieved to be the author of the satirical ro-
mance or collection of satires of which the
15th and i6th books have come down to us,
though in a fragmentary state. The Satyricon
of Petronius, of which the Cena Trimalch-
ionis is the chief piece, gives a vivid picture
of the first century on its seamiest side, and
in style touches the high-water mark of
silver-age Latinity.

Petropavlovsk, town, in Autonomous Ka-
zak Socialist Soviet Republic, 175 m. w. of
Omsk; p.31,000.

Petropolis, town and summer residence,
state of Rio de Janeiro, Brazil; 28 miles n.
of Rio de Janeiro. It is beautifully situated
in the valley of the Organ Mountains, at an
elevation of 2,300 ft. It was originally a col-
ony of Germans (1845), and superseded
Nichtheroy as capital of the state of Rio de
Janeiro from 1893 to 1903. Beer, cheese,
cigars, and cotton goods are manufactured;
p. 30,000.

Petrovsk, town of Soviet Russia; 60 m.
northwest of Saratov city. It has tanneries,
distilleries, breweries, oil and brick works;
p. 19,000.

Petrozavodsk, the capital of the Karelian
Republic of Soviet Russia; 190 miles north-
east of St. Petersburg, on the western shore
of Lake Onega. It manufactures iron and
copper ware; p. 27,000,

Pettenkofer, Max von (1818-1901), Ger-
man chemist, was born near Neubcrg, Ba-
varia. He made many valuable contributions
to science on subjects as various as gold re-
fining, gas making, ventilation, clothing, the
influence of soils on health, epidemics, and
hygiene generally, In particular, his re-
searches laid the foundation of the science
of experimental hygiene. He founded
(1883) and edited the Archiv jiir Hygiene,
He also made notable researches on cholera.

Petty Officer*, Naval, are comparable in